mpp: Add the presplit MPP modifier

Changelog-Added: The MPP presplit modifier splits large payments into 10k satoshi parts to maximize chances of performing the payment and to obfuscate the overall amount being sent.

plugins/pay.c

@@ -1841,6 +1841,7 @@ struct payment_modifier *paymod_mods[] = {
 	&directpay_pay_mod,
 	&shadowroute_pay_mod,
 	&exemptfee_pay_mod,
+	&presplit_pay_mod,
 	&routehints_pay_mod,
 	&waitblockheight_pay_mod,
 	&retry_pay_mod,
@@ -1861,6 +1862,7 @@ static struct command_result *json_paymod(struct command *cmd,
 	const char *label;
 	unsigned int *retryfor;
 	u64 *riskfactor_millionths;
+	struct shadow_route_data *shadow_route;
 #if DEVELOPER
 	bool *use_shadow;
 #endif
@@ -1949,8 +1951,12 @@ static struct command_result *json_paymod(struct command *cmd,
 	p->constraints.cltv_budget = *maxdelay;
 
 	payment_mod_exemptfee_get_data(p)->amount = *exemptfee;
+	shadow_route = payment_mod_shadowroute_get_data(p);
+
+	/* This is an MPP enabled pay command, disable amount fuzzing. */
+	shadow_route->fuzz_amount = false;
 #if DEVELOPER
-	payment_mod_shadowroute_get_data(p)->use_shadow = *use_shadow;
+	shadow_route->use_shadow = *use_shadow;
 #endif
 	p->label = tal_steal(p, label);
 	payment_start(p);

tests/test_pay.py

@@ -3055,3 +3055,27 @@ def test_pay_peer(node_factory):
     # Next one should take the alternative, but it should still work
     inv = l2.rpc.invoice(amt.millisatoshis, "final", "final")['bolt11']
     l1.rpc.dev_pay(inv, use_shadow=False)
+
+
+def test_mpp_presplit(node_factory):
+    """Make a rather large payment of 5*10ksat and see it being split.
+    """
+    MPP_TARGET_SIZE = 10**7  # Taken from libpluin-pay.c
+    amt = 5 * MPP_TARGET_SIZE
+
+    # Assert that the amount we're going to send is indeed larger than our
+    # split size.
+    assert(MPP_TARGET_SIZE < amt)
+
+    l1, l2, l3 = node_factory.line_graph(
+        3, fundamount=10**8, wait_for_announce=True,
+        opts={'wumbo': None}
+    )
+
+    inv = l3.rpc.invoice(amt, 'lbl', 'desc')['bolt11']
+    p = l1.rpc.pay(inv)
+
+    assert(p['parts'] >= 5)
+    inv = l3.rpc.listinvoices()['invoices'][0]
+
+    assert(inv['msatoshi'] == inv['msatoshi_received'])